When wafers are undergoing semiconductor processing in a CVD-type processing chamber 20, as shown in FIGS. 1 and 2, a susceptor 34 faces a gas distribution plate 19 through which process gas flows. Gas products deposit on a wafer 32 supported by the susceptor 34. To prevent the process gas from leaking around the wafer and susceptor 34, a pumping plate 35 closely surrounds the susceptor opening. A circular gap 34a exists between the pumping plate 35 and the edge of the susceptor 34. To prevent process gas from migrating down through this circular slot or gap to coat the backside of the susceptor, purge gas is introduced below the susceptor 34 to flow upward through the slot 34a. The purge gas is distributed around a center portion 21 of the processing chamber in a circumferential chamber purge channel 22 which faces a fused quartz window 38 through which light is transmitted for heating the back of the susceptor 34. The quartz window 38 is placed against the bottom of the processing chamber 20 and is sealed to it by an O-ring seal 36.
During processing, purge gas flows into a purge gas feed passage 29 from outside the processing chamber. This purge gas feed passage 29 terminates in the chamber purge channel 22 (FIG. 2). The purge channel 22 describes an annular ring shaped passage around the edge of the fused quartz window 38. As purge gas passes into the purge channel 22, it passes along the circumferential path of the channel and through the opening of the slotted orifice 43 between the fused quartz window 38 and the inner wall of the chamber purge channel 22. The gas then flows up towards and through the gap 34a between the susceptor 34 and the pumping plate 35 and flows towards holes 35a in the pumping plate until it reaches a pumping channel 41 (FIG. 2). The pumping channel 41 is connected to the vacuum system which evacuates the processing chamber during processing.
The gap 43 between the processing chamber wall 20a adjacent to the chamber purge channel 22 and the surface of the fused quartz window 38 describes a circular slot. A continuous circular orifice between the chamber purge cell 22 and the center portion of the processing chamber 21. The size of this gap 43 can depend on the resiliency of the O-ring seal 36 or may be set by a teflon seal 66 such as shown in FIG. 5A. However, with a long, continuous orifice fed only at one point gas distribution through the slot along the slot may vary even with a constant purge flow because of the nature of the flow; sometimes it is necessary to change process gas flows and pressures for different processes which can vary the purge gas flow pattern unpredictably. Similarly purge gas flows must be changed to assess and utilize more efficient processing protocols. So, while one particular slot opening will work at a particular flow rate and pressure differential, a change in the flow rate or a change in the pressure differential can adversely and unpredictably affect the location at which gas molecules exit the chamber purge channel 22 and flow into the central portion 21 of the processing chamber 20.
A purge flow whose flow rate has been reduced such that uniform pressure is not found in the chamber purge channel at all locations around its circumference can produce a flow pattern similar to that which is visually depicted in FIG. 1. The arrows 30 can be understood to be nearly equal units of gas flow. Substantially more gas flow occurs closely adjacent to the location of the purge gas feed passage 29 (indicated by the greater numbers of arrows close to the feed passage) and less gas flows (the gas flow arrows 30 being spaced farther apart) from the orifice at process chamber locations opposite the purge gas feed passage 29. The flow as described and depicted in FIG. 1 tends to produce a distortion in the process gas flow as the purge gas tends to displace the process gas non-uniformly at predetermined gas flows. Distortion of the process gas flow pattern reduces the uniformity of the deposited material on a wafer surface requiring extra coating thickness allowances to overcome such non-uniformities.